The present invention relates to a tape address synchronizing apparatus for a video tape recorder (VTR) which equalizes a tape address of the VTR with that of another video tape recorder or with a reference.
Such tape address synchronizing operation is necessary for editing a video program, for example. When the video program is edited into a recording VTR by receiving the video program from a playing VTR, both the recording VTR and the playing VTR rewind their magnetic tape by a predetermined time (pre-roll time) from a certain edit point, first, and, then, both the VTRs turn on in the normal direction. After this, the editing starts at the edit point. However, the edit point in both the VTRs, i.e., the recording VTR and the playing VTR, frequently does not coincide with the tape address synchronizing operation because of a difference in pre-roll times actually set up into the recording VTR and the playing VTR and a difference in mechanical characteristics of the two VTRs. The tape address synchronizing operation equalizes the tape address, i.e., the edit point during the pre-roll time. One method of the tape address synchronizing operation is that a tape address of either the recording VTR or the playing VTR is employed as a reference, and the reference is transmitted to the other VTR which controls its tape speed in order to lock-in with the reference. Another method is that an external reference is applied to both the recording VTR and the playing VTR and the two VTRs control their tape speed in order to lock-in with the external reference. The following description is based on the latter method.
In a prior art tape address synchronizing apparatus for a VTR, a reference address generation circuit, which can be loaded with a preset address, produces a reference address in accordance with an external reference, and the reference address is compared with a tape address which is produced in response to actual tape transportation. The result from the comparison is fed into a capstan servo circuit and a feedback arrangement is used to ensure that the comparison result is driven to zero. In addition, the capstan servo circuit also controls a capstan motor to cause a control track (CTL) signal, which is recorded on a control track of a magnetic tape, coincide with the external reference. Accordingly, after the capstan servo circuit has operated so that there is no difference between the reference address and the tape address, it also makes the CTL signal coincident with the reference. However, since the servo response due to comparing the addresses is independent of the response based on comparing the CTL signal with the reference, the address difference is often changed after the servo operation which aligns the CTL signal with the reference. In such case, both servo operations must be repeated. In consequence the tape address synchronizing operation cannot be completed during the pre-roll time. As a result, accurate editing cannot be accomplished.